I would agree with your assessment. No point, since your couch is against the rear wall -- exactly the same setup as I have. When I tested my setup with temporary installation of a 7.1 arrangement, I could not detect any significant (i.e. audible) gains over 5.1.

I moved my couch out a couple of feet from the rear wall and it sounded better. I doubt that you'd want to leave it this way in your current arrangement, but you might try it just to see if it improves the sound field any.